Fluid-filled equipment, which includes transformers, cables, bushings, terminations and circuit breakers, is an integral part of the electrical system. This diverse equipment represents considerable utility investment and a high proportion of it is advancing in age. About 30% of the high pressure fluid-filled transmission cable systems, the predominant form of U.S. underground transmission, are over 25 years old. Likewise, about 35% of the power transformers are over 30 years old. The rapidly emerging utility business climate dictates efficient use of such assets. Among other measures, this dictates proper periodic maintenance. The assessment of the condition of fluid-filled equipment through traditional evaluations of a fluid sample (dielectric breakdown, dissipation factor and color, etc.) has been made since the introduction of such equipment. This does not lead to reliable results for in-service fluids, although these tests are most appropriate for the selection and evaluation of new dielectric fluids. To enhance the value of periodic fluid testing, newer tests such as Dissolved Gas Analysis (DGA) and furfural content have been increasingly applied to fluid-filled equipment, with promising results.
The conventional method relating to the dissolved gas analysis of oil-filled electrical equipment is described in ASTM D3612. As a first step of the analysis, the oil sample is taken from a service cable with a glass syringe or a stainless steel cylinder and eventually brought to the laboratory where the gases are extracted from the oil under vacuum. In the second step, the extracted gas is collected in a second glass syringe and then taken to a gas chromatograph for the determination of individual component gases. While this procedure seems to serve well, it has several drawbacks, and it cannot be automated to increase sample throughput. The introduction of a second glass syringe contributes to the overall experimental error. The sampling of gas from the extraction apparatus and its subsequent injection to the gas chromatograph contributes to overall experimental error.
More recently, a method of analysis in which the gases in the head space of a vial are analyzed has been developed. In this approach, both the fluid sampling and analysis are performed in the same vial, leading to reduced sample manipulation and consequent enhanced accuracy and precision. This method is described in EPRI Final Report EL-7488-L, November, 1991. While the prior art system has many advantages and has served the industry well, the time required to assemble, disassemble and clean the cells, despite an efficient cleaning system involving a vapor degreaser, adds to the cost of analysis. The analysis cells are large, heavy and expensive. Shipping containers are heavy and add to the cost of shipping the samples for analysis.